1. Field of the Invention
The present invention relates to an interphone system, and more particularly to one which utilizes existing telephone sets and televisions to communicate and monitors a visitor.
2. Description of the Prior Art
Interphone systems are commonly used in multi-story buildings, especially apartment buildings. Most advanced interphone systems include an outdoor phone set installed in the lobby door, a plurality of indoor interphone sets each of which is installed in a corresponding apartment, a monitor installed in a corresponding apartment, and an electric lock control device installed in each indoor interphone set. Usually the indoor interphone set, the monitor, and the electric lock control device are installed together beside a front door of each apartment. Therefore, a resident (user) can see a visitor from the monitor, communicate with the visitor by the indoor interphone set, and decide whether or not to open the lobby door by operating the electric lock control device. The conventional interphone system works well but has a high cost. For example, the conventional interphone system requires many telephone lines for interconnecting the outdoor set with all the indoor interphone sets and many monitors for all the apartments. However, each apartment could utilize its telephone set and television set to replace the indoor interphone set and the monitor, thereby reducing costs.